The present invention relates to the sector of radio signal transmission and more precisely to a method and a relative circuit adapted to transfer the angle modulation of an intermediate frequency to a microwave carrier using a PLL which locked band is automatically adapted to the bandwidth of the intermediate frequency signal. The invention will be hereafter described for the use in telecommunication equipment such as for example in small capacity digital radio links according to a manufacturing sector the applicant is involved in, but it is however understood that the same invention may be advantageously applied in other electronic engineering sectors.
In radio signal transmitters is universally employed the heterodyne technique that foresees a first conversion at intermediate frequency IF of the baseband signal, the filtering, and a further radio frequency conversion in the specific transmission band of the preselected channel. Analogously, in reception the syntonized channel is first of all converted into intermediate frequency IF, filtered, and then converted in baseband by demodulation. The advantages of this technique are well known. The frequency conversion, both up and down, is carried out transmitting the signal to be converted to a device presenting a not linear transfer characteristic, for example a diode, and piloting conductiont of said device by means of sinusoidal pump signal generated by a local oscillator at a frequency differing from that of the center-band of the radio frequency channel for xc2x1the value of the intermediate frequency. Practically the pump signal is generated by a local Voltage Controlled Oscillator or VCO, inserted in a Phase Locked Loop or (PLL), which is reached by a reference signal provided with great stability in frequency. The functioning of the PLL structures is well-known by those skilled in the art, but it is however opportune to underline the fact such structures comprise generally a frequency divider applied at the output of the VCO to enable the use of reference clock signals with absolutely lower frequencies compared to that of the signal generated by the VCO.
A fundamental problem arising for a company manufacturing professional transmission systems as for example those used in the stations of the sections connected by radio links, is that to achieve excellent functioning performances maintaining however the production cost low. A first difficulty in this sense is given by the necessity to be obliged to employ microwave mixers. A second limitation is given by the difficulty in realising the PLL for the piloting of the aforesaid mixers with very high frequencies, which are in small capacity digital radio links in the range of 15 GHz.
A first solution realised by the applicant to reduce the manufacturing costs at the expenses of an imperceptible reduction of the performances, consists in the use of standard glass reinforced FR4 printed circuits as microwave substrate. Such solution is disclosed in International Application no. PCT/EP98/04028 in the name of the same applicant. The object of the above-mentioned patent application is in fact a microwave VCO realised in planar technology, which layout is obtained on the same fibreglass substrate on which are realised circuits with lower frequency for the control of the VCO. Said new approach has it made possible to reduce the high production costs, thanks to the construction simplification of the circuit modules.
The applicant has recently accomplished a further step forward succeeding to eliminate the mixer used in the transmitters to convert the intermediate frequency signal to microwave. This has been possible by using the VCO, no more to generate the pumping signal, clearly useless if there is no mixer, but rather to generate directly the transmission signal. The innovation has not required outstanding circuit variations in the PLL generating the local oscillator signal, as it has been sufficient enough to replace the previous reference signal for the phase comparator by the intermediate frequency signal to be transmitted. In this way the phase modulation the intermediate frequency signal is affected by will be microwave directly transferred from the VCO of the PLL.
This second solution, that makes use of the embodiment advantages of the first solution, is described in the article entitled xe2x80x9cFR4 PCB MODULATION TRANSFER MW VCO UP TO 16 GHZxe2x80x9d, authors C. Buoli, G. Mora and L. A. Cervi, published after the Proceedings of the 1997 27th European Microwave Conference. Part 2. Jerusalem, Sep. 8-12, 1997.
FIG. 1 shows a circuit scheme of a PLL 1 realising both known solutions, even if only the second will be considered. The PLL 1 is usefully employed in the low capacity digital microwave radio system transmitters.
With reference to FIG. 1 it is possible to state that the PLL 1 comprises it comprehends a VCO 2, essentially set up by an active element 3 electronically piloted by means of a varactor 4. The signal coming out of the VCO 2 is sent to a microwave buffer 5 placed uphill of a directional coupler 6 distributing the input signal between two output gates. A first of these signals in output from the coupler 6 is sent to a balanced buffer 7 in such a way to obtain in output a transmission radio frequency RF with a right power level and a good degree of isolation. A second signal Si is sent dispatched, by means of a further microwave buffer 8 to a sampling phase detector 9, or SPD which is at its turn fed by a sinusoidal signal S2 generated at a suitable frequency UHF. Said signal is advantageously obtained by means of a VCO at low phase noise. At the output of the phase detector 9, a signal S3 is present, copy of the signal RF translated at intermediate frequency, that reaches a first input of a phase comparator 10 set up by a mixer, at which second input a reference signal S4 arrives, correspondent to an intermediate frequency signal obtained by a modulated phase signal from digital data. At the output of the phase comparator 10 there is therefore an error signal S5 for the control of the VCO 2. This signal passes first of all a loop broad band filter 11, useful to eliminate undesired spurious radiation produced in the PLL 1, the latter being set up by the VCO 2, the phase detector 9, the phase comparator 10, and the loop filter 11. It is important to note the fact that the larger the lock band of PLL 1 is, the higher is its capacity to maintain the lock even if there are sudden phase variations in the modulated reference signal. The loop gain necessary for the good functioning of the PLL is guaranteed by the syntonisation capacity of the oscillator VCO 2 (about 150 MHz/VOLT) and by the sensitiveness of the phase comparator 10 (300/2xcfx80 mV/rad). In order to enable the locking of PLL 1 to the reference signal during the lighting phase or after a possible out of lock condition of this one, the insertion of an automatic research device of the locking is foreseen on the way connecting the phase comparator 10 to the loop filter 11. For simplicity reasons said known device is not shown in the figure. Regarding the phase detector 9, one of its possible embodiments will be described in details discussing FIG. 2.
Compared to the already known art where the PLL was used to lock at an exact reference the pump frequency of the microwave mixer, the same PLL will now be used in a different way to transfer directly to microwave the phase modulation the intermediate frequency reference carrier is affected by. Therefore the distinctive characteristic is evident between both known solutions just illustrated; it consists in the fact that while in the first case the mixer transfers (converts) in frequency a modulated signal, regenerating it to microwave through a beat, in the second case the PLL copies in the microwave carrier generated by itself the phase modulation of the reference signal, and therefore without the need to transfer the reference signal by means of a beat.
The generality in the transfer of the modulation concerns the so-called angle modulation that is of the phase or the frequency, both of the analogue and digital type, while the amplitude modulation remains obviously excluded. As admitted digital modulations we quote for example the well-known modulations M-FSK, M-PSK, CPM (Continuous Phase Modulation) etc., while the mixed amplitude and phase modulations of the type M-QAM are excluded. As already known there is a mathematical relation between frequency and phase, and therefore between the homonymous modulations, the frequency being nothing else but an instant phase variation, that is the time derivation of the phase.
In the case of the example of FIG. 1 a digital signal is used to operate the CPM 1 modulation of a carrier, so to be able to benefit from the advantages offered by such modulation that, maintaining the phase continuity between two intervals of contiguous signalling, generates extremely contained and constant amplitude spectrum of the modulated signals. All this is translated in the concrete opportunity to save transmission power thanks to the use of radio frequency power amplifiers functioning in class C, followed by a band-pass filter that extracts from the amplified signal only the band of interest.
As already known the amplifiers in class C present the highest performance. With the mixed digital modulations mentioned before it is instead necessary to use some absolutely linear amplifiers in class A, in order to avoid the introduction into the band of the amplified signal any distortions of type AM/AM and AM/PM.
The skilled in the art could probably consider the PLL 1 like a simple angle modulator, but this is not completely correct, even if at a first glance acceptable, because the aforesaid angle modulation pre-exists to the following treatment by the PLL, to which it can only be adapted, but at a higher carrier frequency. Modulation of a signal requires commonly sophisticated realisation schemes, which a PLL is certainly not able to reproduce in case the use of it was wanted to modulate the microwave carrier since the beginning. It is therefore more reasonable to talk about angle modulation transfer, in line with title of the article mentioned in relation to the realisation of a phase modulation conveyor realised by the same applicant from which the invention to be described differs for its originality.
Although all the advantages pointed out until now, the circuit of FIG. 1 may nevertheless be improved regarding the quality of the generated signal. And this is the main object of the present invention.
The aforesaid circuit in fact does not perform in an optimal way with respect to noise, if, for system convenience reasons it might be advantageous to choose the limitation by a single lock band of the PLL all possible bit-rates foreseen in the digital radio system in which said circuit is used. As an example the foreseen transmission speeds are the following: 2xc3x972 Mbit/sec, 4xc3x972 Mbit/sec, 8xc3x972 Mbit/sec, and 16xc3x972 Mbit/sec. Supposing to vary occasionally the transmission speed for changed traffic requirements maintaining nevertheless unchanged the particular digital modulation initially predetermined, will than vary according the transmission band. Obviously the PLL lock band has to be sized for the higher transmission speed and in this way it will turn out to be oversized for the minor bit-rates. Due to the oversize the spurious and harmonic ones produced by the signal IF or generated by the modulation system itself are captured by the PLL and amplified.
In case instead the bit-rate of the modulators used in all stations of a radio system has been predefined for all of them during the engineering step, the inconvenience just pointed out could be avoided by optimising the passband of the PLL according to the bit-rate. But in this case there would be anyhow the problem to be obliged to keep the same number of modulation conveyor circuits in the catalogue as that of the bit-rates foreseen between the terminal stations of a determined radio system.
But the lock band differs also according to the variation of certain physical parameters of the devices used in the PLL, i.e. for example it varies according to the sensitivity of the VCO with the functioning temperature of the equipment that they have to be able to tolerate variations comprised between xe2x88x9240xc2x0 and +75xc2x0 according to the installation sites, it may vary due to the ageing of the devices setting up the PLL and also according to the frequency shifting to the right or the left of the carrier given that the system is only in theory perfectly symmetric. All this requires to foresee an ample margin of security and to use bands which are largely oversized compared to the actual transmission needs which emphasises the above-mentioned drawbacks (especially at the lowest bit-rates), and it makes the respect of the functioning masks approuved by international standardization autorities for the approval of the single systems even more difficult.
The previous considerations may be extended, with the due distinctions, even regarding the angle modulations of the analog type.
Referring to what said before, a transmission system using the angle modulation conveyor realised according to the circuit scheme of FIG. 1 presents the drawback of not being able to maintain unchanged the quality of the signal transmitted with the variation of the use and installation conditions. Therefore the technical problem deriving from that is that to obtain in an absolutely automatic way a constant and optimal quality of the transmitted signal even if the above-mentioned conditions change or, said in an equivalent way, a lock band of the angle modulation conveyor PLL limiting itself in an adaptive way to the value of the band of the reference signal. Under this introductory statement, the spurious an harmonics produced by the intermediate frequency or generated by the modulation system itself will be automatically filtered by the PLL, and the noise will turn out reduced as a whole all to the advantage of the spectral purity.
The present invention offer a solution to the above-mentioned technical problem as it is addressed to a method adapted to transfer to a transmission frequency carrier the angle modulation of a carrier at a lower frequency used by a phase lock mesh, or PLL, as a modulated reference signal to generate said radio frequency carrier by means of a voltage controlled oscillator or VCO, the lock band of the PLL being susceptible of band width variations due to changed traffic requirements, thermic variations, ageing of the used electronic devices.
Said method including the following steps of:
a) frequency conversion of the signal generated by the VCO at said lower frequency;
b) phase comparison between said signal converted at a lower frequency and said modulated frequency signal to achieve an error signal for the control of the VCO;
characterised in that it includes furthermore the steps of:
c) detection of the peak value of said error signal;
d) generation of a regulation signal of the lock band of said PLL as the difference between said peak value and a predetermined reference value;
e) attenuation of the level of said error signal in relation to the value of said regulation value of the lock band of the PLL, maintaining in this way the relation K between the lock band of the PLL and the band width of said modulated reference signal, as described in claim 1.
Favourably said radio frequency carrier occurs during the interval of the microwaves and said carrier with a lower frequency has a suitable intermediate frequency. Nevertheless, in a principle, it is not possible to exclude practical subjects, where the modulation transfer may occur directly from base-band to microwave.
A further object of the present invention is a conveyor circuit with angle modulation radio frequency, based on a PLL which functioning is in compliance to the method illustrated before and disclosed in claim 13.
The PLL of the modulation conveyor circuit object of the present invention has the advantage to have a lock band limited in an adaptive way to the value of the band width of the reference signal, which for a signal modulated in the analog way depends on the modulation index and on the peak amplitude value of peak of the modulating signal, while for a digitally modulated signal this depends on the bit-rate of the modulating signal and on the predetermined modulation scheme, as well as on the shape of the modulating pulse.
Thanks to the method proposed by the instant inventors, the PLL of the conveyor circuit presents the further advantage to have a lock band, which is independent, both of the temperature of the environment in which the equipments are working and of the ageing of the devices. In fact, the bandwidth of the reference signal being generally completely independent on the thermic variations and on the ageing, also the lock band of the PLL turns out being dragged due to the constancy of the relation K maintained between these two bands. Therefore the two possible adjustments are unnecessary which otherwise would have to be carried out on the installation sites of the equipment, which are quite often difficult to be reached. Another advantage, which cannot be neglected, is that of avoiding the maintenance in production of different versions of the same circuit for the need of optimising the performances at the different bit-rates.